This invention relates to the recovery of liquid and gaseous products from an in situ oil shale retort in a subterranean formation.
The presence of large deposits of oil shale in the high plateau, semi-arid region of the western United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; and 4,043,598 which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded to form a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil as described in U.S. Pat. No. 3,661,423 includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products and a residual solid carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
U.S. Pat. No. 4,043,598 discloses a method for explosively expanding formation containing oil shale toward horizontal free faces to form a fragmented mass in an in situ oil shale retort. According to a method disclosed in that patent, a plurality of vertically spaced apart voids of similar horizontal cross-section are initially excavated one above another within the retort site. A plurality of vertically spaced apart zones of unfragmented formation are temporarily left between the voids. Explosive is placed in each of the unfragmented zones and detonated, preferably in a single round, to explosively expand each unfragmented zone into the voids to form a fragmented mass. Retorting of the fragmented mass is then carried out to recover shale oil from the oil shale.
U.S. Pat. No. 4,043,596 discloses a method of forming an in situ oil shale retort in a subterranean oil shale deposit by excavating at least one vertically extending columnar void. The surface of the formation which defines the columnar void presents at least one free face which extends vertically through the subterranean oil shale deposit. A portion of the formation within the boundary of the in situ retort to be formed and which extends away from the free face is explosively expanded toward the columnar void in one or more segments. The expansion of the oil shale toward the columnar void fragments the oil shale, thereby distributing the void volume of the columnar void throughout the retort.
It is desirable to form an in situ retort with a generally uniformly distributed void fraction having a fragmented mass of generally uniform permeability so that oxygen-supplying gas can flow relatively uniformly through the fragmented mass during retorting operations. Techniques used for explosively expanding zones of unfragmented formation toward a free face of formation adjacent a void can control the uniformity of particle size and permeability of the fragmented mass. A fragmented mass having generally uniform permeability in horizontal planes across the fragmented mass avoids bypassing portions of the fragmented mass by retorting gas.
In blasting to a vertical void, a present method is to drill rows of substantially vertical blastholes in the formation to be expanded wherein the blastholes are substantially equal in diameter and each row has about the same burden distance. Burden distance as used herein is the substantially perpendicular distance from the center of mass of an explosive charge in such a row to the free face toward which the unfragmented formation is being expanded.
One of the problems of using a blast design with equal size blastholes spaced in rows having about equal burden distance for blasting to a void is the formation of a fragmented mass having an unevenly distributed void fraction. The uneven distribution is caused by the decreasing volume of void space that is available for expansion of each subsequent layer of rock as the blast proceeds away from the initial free face. This method results in maximum void fraction in the region of the vertical void and less void fraction, i.e., tighter rubblization, in the outer portions of the retort away from the vertical void.
Having a retort with a gradient of void fraction across the horizontal cross-section of the fragmented permeable mass of oil shale particles can cause gas channeling, bypassing of oil shale by the retorting zone, and a resultant reduction in yield of gaseous and liquid products from the retort.